A light-water nuclear reactor has a core of nuclear fuel which is cooled by recirculating water. A reactor pressure vessel contains the reactor coolant. Piping circuits carry the heated water or steam to the steam generators or turbines and carry circulated water or feedwater back to the vessel. Operating pressures and temperatures for the reactor pressure vessel are about 7 MPa and 288.degree. C. for a boiling water reactor (BWR), and about 15 MPa and 320.degree. C. for a pressurized water reactor (PWR). The materials used in both BWRs and PWRs must withstand various loading, environmental and radiation conditions.
Some of the materials exposed to high-temperature water include carbon steel, alloy steel, stainless steel, and nickel-based, cobalt-based and zirconium-based alloys. Despite careful selection and treatment of these materials for use in water reactors, corrosion occurs on the materials exposed to the high-temperature water. Such corrosion contributes to a variety of problems, e.g., stress corrosion cracking, crevice corrosion, erosion corrosion, sticking of pressure relief valves and buildup of the gamma radiation-emitting Co-60 isotope.
Stress corrosion cracking (SCC) is a known phenomenon occurring in reactor components, such as structural members, piping, fasteners, and welds, exposed to high-temperature water. As used herein, SCC refers to cracking propagated by static or dynamic tensile stressing in combination with corrosion at the crack tip. The reactor components are subject to a variety of stresses associated with, e.g., differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stress from welding, cold working and other asymmetric metal treatments. In addition, water chemistry, welding, heat treatment, and radiation can increase the susceptibility of metal in a component to SCC.
It is well known that SCC occurs at higher rates when oxygen is present in the reactor water in concentrations of about 5 ppb or greater. SCC is further increased in a high radiation flux where oxidizing species, such as oxygen, hydrogen peroxide, and short-lived radicals, are produced from radiolytic decomposition of the reactor water. Such oxidizing species increase the electrochemical corrosion potential (ECP) of metals. Electrochemical corrosion is caused by a flow of electrons from anodic to cathodic areas on metallic surfaces. The ECP is a measure of the thermodynamic tendency for corrosion phenomena to occur, and is a fundamental parameter in determining rates of, e.g., SCC, corrosion fatigue, corrosion film thickening, and general corrosion.
In a BWR, the radiolysis of the primary water coolant in the reactor core causes the net decomposition of a small fraction of the water to the chemical products H.sub.2, H.sub.2 O.sub.2, O.sub.2 and oxidizing and reducing radicals. For steady-state operating conditions, equilibrium concentrations of O.sub.2, H.sub.2 O.sub.2, and H.sub.2 are established in both the water which is recirculated and the steam going to the turbine. These concentrations of O.sub.2, H.sub.2 O.sub.2 and H.sub.2 can result in conditions that promote intergranular stress corrosion cracking (IGSCC) of susceptible materials of construction.
As used herein, the term "critical potential" means a corrosion potential at or below a range of values of about -230 to -300 mV based on the standard hydrogen electrode (SHE) scale. IGSCC proceeds at an accelerated rate in systems in which the ECP is above the critical potential, and at a substantially lower or zero rate in systems in which the ECP is below the critical potential. Water containing oxidizing species such as oxygen and hydrogen peroxide increase the ECP of metals exposed to the water above the critical potential, whereas water with little or no oxidizing species present results in an ECP below the critical potential.
Thus, susceptibility to SCC in BWRs is highly influenced by corrosion potential. FIG. 1 shows the observed and predicted crack growth rate as a function of corrosion potential for furnace-sensitized Type 304 stainless steel at 27.5 to 30 MPa.sqroot.m in 288.degree. C. water over the range of solution conductivities from 0.1 to 0.5 .mu.S/cm. Data points at elevated corrosion potentials and growth rates correspond to irradiated water chemistry conditions in test or commercial reactors. Reduction of the corrosion potential is the most widely pursued approach for mitigating SCC in existing plants.
One method employed to mitigate IGSCC of susceptible material is the application of hydrogen water chemistry (HWC), whereby the oxidizing nature of the BWR environment is modified to a more reducing condition. This effect is achieved by adding hydrogen gas to the reactor feedwater. When the hydrogen reaches the reactor vessel, it reacts with the radiolytically formed oxidizing species on metal surfaces to reform water, thereby lowering the concentration of dissolved oxidizing species in the water in the vicinity of metal surfaces. The rate of these recombination reactions is dependent on local radiation fields, water flow rates and other variables.
The injected hydrogen reduces the level of oxidizing species in the water, such as dissolved oxygen, and as a result lowers the ECP of metals in the water. However, factors such as variations in water flow rates and the time or intensity of exposure to neutron or gamma radiation result in the production of oxidizing species at different levels in different reactors. Thus, varying amounts of hydrogen have been required to reduce the level of oxidizing species sufficiently to maintain the ECP below the critical potential required for protection from IGSCC in high-temperature water.
It has been shown that IGSCC of Type 304 stainless steel used in BWRs can be mitigated by reducing the ECP of the stainless steel to values below -0.230 V(SHE). However, high hydrogen additions, e.g., of about 200 ppb or greater, that may be required to reduce the ECP below the critical potential, can result in a higher radiation level in the steam-driven turbine section from incorporation of the short-lived N-16 species in the steam. Thus, recent investigations have focused on using minimum levels of hydrogen to achieve the benefits of HWC with minimum increase in the main steam radiation dose rates.
An effective approach to achieve this goal is to either coat or alloy the stainless steel surface with palladium or any other platinum group metal. As used herein, the term "platinum group metal" means metals from the group consisting of platinum, palladium, osmium, ruthenium, iridium, rhodium, and mixtures thereof. The presence of palladium on the stainless steel surface reduces the hydrogen demand to reach the required IGSCC critical potential of -0.230 V(SHE). Compared to the HWC technique, which employs large hydrogen additions to suppress and recombine oxygen and hydrogen peroxide formed by radiolysis to very low levels (e.g., &lt;2 ppb), the noble metal approach requires only that sufficient hydrogen be present so that, as water is formed on the catalytic surface, all oxygen and hydrogen peroxide are consumed (e.g., 2H.sub.2 +O.sub.2 .fwdarw.2H.sub.2 O). Additionally, lower potentials (generally the thermodynamic minimum) are obtained. Depending on the precise location within a BWR, the hydrogen addition required in the noble metal approach is reduced by a factor of 5 to 100.
The fundamental importance of corrosion potential versus, e.g., the dissolved oxygen concentration per se is shown in FIG. 2, where the crack growth rate of a crack growth specimen coated with palladium by electroless plating drops dramatically once excess hydrogen conditions are achieved, despite the presence of a relatively high oxygen concentration. FIG. 2 shows plots of crack length and corrosion potential vs. time for a Pd-coated crack growth specimen of sensitized Type 304 stainless steel showing accelerated crack growth in 288.degree. C. water containing excess oxygen (e.g., 1000 ppb O.sub.2 and 48 ppb H.sub.2). Because the crack growth specimen was Pd-coated, the change to excess hydrogen (e.g., 400 ppb O.sub.2 and 78 ppb H.sub.2) caused the corrosion potential and crack growth rate to drop.
U.S. Pat. No. 5,135,709 to Andresen et al. discloses a method for lowering the ECP on components formed from carbon steel, alloy steel, stainless steel, nickel-based alloys or cobalt-based alloys which are exposed to high-temperature water by forming the component to have a catalytic layer of a platinum group metal. This layer catalyzes the recombination of reducing species, such as hydrogen, with oxidizing species, such as oxygen or hydrogen peroxide, that are present in the water of a BWR. Such catalytic action at the surface of the alloy can lower the ECP of the alloy below the critical potential where IGSCC is minimized. As a result, the efficacy of hydrogen additions to high-temperature water in lowering the ECP of components made from the alloy and exposed to the injected water is increased manyfold. Furthermore, it is possible to provide catalytic activity at metal alloy surfaces if the metal substrate of such surfaces contains a catalytic layer of a platinum group metal. A solute can be provided by methods known in the art, for example by addition to a melt of the alloy or by surface alloying. Alternatively, a coating of platinum group metal provides a catalytic layer and catalytic activity at the surface of the metal. Suitable coatings can be deposited by methods well known in the art, such as plasma spraying, flame spraying, chemical vapor deposition, physical vapor deposition processes such as sputtering, welding such as metal inert gas welding, electroless plating, and electrolytic plating. However, these approaches are ex-situ techniques in that they cannot be practiced while the reactor is in operation.
The development of techniques to apply palladium in situ to all wetted components represents a breakthrough in extending the applications of the noble metal technology, since manual application (e.g., by thermal spray or fusion cladding) requires complex tooling, is slow and expensive, and can only coat surfaces to which there is sufficiently good access. U.S. patent applications Ser. Nos. 08/143,513 and 08/209,175 disclose a technique to coat or dope oxidized stainless steel surfaces in situ by injecting a metal-containing compound into the high-temperature water, which metal has the property of improving the corrosion resistance of those surfaces. The compound is injected in situ in the form of a solution or a suspension. The preferred compound for this purpose is palladium acetylacetonate, an organometallic compound. The concentration of palladium in the reactor water is preferably in the range of 5 to 100 ppb. Upon injection, the palladium acetylacetonate decomposes and deposits palladium on the oxidized surface. Palladium may be deposited within or on the surface of the oxide film in the form of a finely divided metal. The oxide film is believed to include mixed nickel, iron and chromium oxides.
The ECPs of the stainless steel components should all drop by -300 mV after palladium injection. It is possible to reduce the ECP of Type 304 stainless steel to IGSCC protection values without injecting hydrogen provided that organics are present in the water. This occurs because of the catalytic oxidation of organics on Pd-doped surfaces.
Following palladium injection, hydrogen can be injected into the reactor water. As hydrogen is added, the potential of the Pd-doped oxide film on the stainless steel components is reduced to values which are much more negative than when hydrogen is injected into a BWR having stainless steel components which are not doped with palladium.
Other palladium compounds of organic, organometallic or inorganic nature, as well as compounds of other platinum group metals or non-platinum group metals such as titanium and zirconium, can also be used.
In summary, the oxygen content of the reactor water can be reduced by palladium injection alone initially. Some oxygen will be reduced by the organics of the organometallic palladium compound following thermal decomposition or radiolytic decomposition (induced by gamma and neutron radiation) of the organometallic palladium compound. When palladium injection is combined with hydrogen injection, oxygen will also be reduced as a result of the recombination of dissolved oxygen and hydrogen molecules at the Pd-doped surfaces forming water molecules.
The effectiveness of alloys or coatings that contain at least about 0.1% noble metal (which category of metals is also referred to in the art as "platinum group metals") has been extensively demonstrated. The data presented in FIG. 3 were obtained using pre-oxidized Type 304 stainless steel electrodes held in 288.degree. C. water containing 300 ppb O.sub.2 for 8 months. The presence of platinum reduced the corrosion potential of Type 304 stainless steel for dissolved hydrogen levels in excess of about 24 ppb. The amount of platinum was varied as follows: (.circle-solid.) no Pt; (.diamond-solid.) 0.1% Pt; (.tangle-solidup.) 0.35% Pt; (.box-solid.) 1.0% Pt; (.largecircle.) pure Pt.
In situ palladium deposition from aqueous solutions on pre-oxidized materials has also been shown to be effective, both in terms of deposition (the presence of palladium on the surface has been confirmed by Auger electron spectroscopy and X-ray photoelectron spectroscopy) and catalytic response (in high-temperature water containing stoichiometric excess hydrogen). FIG. 4 shows a plot of crack length and solution conductivity vs. time for a Pd-coated crack growth specimen of furnace-sensitized Type 304 stainless steel showing accelerated crack growth in 288.degree. C. water containing about 180 ppb O.sub.2 and 9.6 ppb H.sub.2. Because the crack growth specimen was Pd-coated (i.e., in shallow water by the high-velocity oxy-fuel technique with Type 309L stainless steel+0.42% Pd), the change to excess hydrogen (i.e., 150 ppb O.sub.2 and 24 ppb H.sub.2) caused the corrosion potential and crack growth rate to drop.
FIG. 5 is a plot of crack length vs. time for a Pd-doped crack growth specimen of furnace-sensitized Alloy 182 weld metal showing accelerated crack growth in 288.degree. C. water containing excess oxygen and reduced crack growth under excess hydrogen conditions in the presence of palladium. Palladium doping was performed on a pre-oxidized (and previously tested) crack growth specimen from a 100 ppb (as Pd) aqueous solution of palladium acetylacetonate. The specimen was first exposed to zinc and then Pd-doped for 48 hr.
However, it has also been shown that exposure to prolonged ultrasonic cleaning significantly reduces both the presence of palladium and the surface catalytic response. FIG. 6 shows a significant reduction in the catalytic effectiveness of in-situ palladium deposition in reducing the corrosion potential in 288.degree. C. water under stoichiometric excess hydrogen conditions following prolonged exposure to ultrasonic cleaning. Deposition was performed on a pre-oxidized coupon specimen from a 100 ppb (as Pd) aqueous solution of palladium acetylacetonate. FIG. 6 shows corrosion potential as a function of H.sub.2 /O.sub.2 molar ratio for the following materials in 288.degree. C. water having 1.0 ppm O.sub.2 : (.circle-solid.) undoped Type 304 stainless steel; (.tangle-solidup.) Pd-doped Type 304 stainless steels doped by in situ deposition and not exposed to ultrasonic cleaning; (.box-solid.) Pd-doped Type 304 stainless steels doped by in situ deposition and then exposed to 60.degree. C. water for 1 week in an ultrasonic bath; and (.largecircle.) pure platinum. The Pd-doped specimen showed a 250 mV increase in ECP after being exposed to ultrasonic cleaning. These results indicate the loss of the surface catalytic property.